D.C. Code § 47-838
The property occupied by the Baltimore and Potomac Railroad Company under authority of this section, together with the improvements which may be put thereon, shall be subject to tax by the District of Columbia the same as other property in the District of Columbia; provided, that no assessment, valuation, or tax shall be made, laid, or levied on the Baltimore and Potomac Railroad Company on account of any bridges, tunnels, elevated tracks, or subway which shall be located, constructed, or maintained under the authority of this act, and forming part of said railroad, in excess of that which would or could be lawfully made, laid, or levied if said railroad was wholly located and constructed on the surface of the ground; it being the true intent and meaning hereof that any such bridges, tunnels, elevated tracks, or subway forming a part of said railroad shall be assessed and valued for purposes of taxation and taxed on the same basis as any other equal portion of railroad situated within the said District of Columbia not constructed on, in, through, or upon any such bridges, tunnels, elevated tracks, or subway.
D.C. Code § 47-838